Polybenzobisthiazole (PBT) and polybenzobisoxazole (PBO) are taught in the prior art as useful in the preparation of high modulus, high strength materials having excellent thermal stability. PBO has been synthesized by the condensation of 4,6-diaminoresorcinol dihydrochloride with terephthalic acid in polyphosphoric acid. PBT has been synthesized by a similar process from 2,5-diamino-1,4-benzenedithiol dihydrochloride and terephthalic acid (see Wolfe, U.S. Pat. No. 4,225,700). The use of terephthalic acid of small particle size is one of the conditions determined in the art to be necessary to afford PBT with high inherent viscosity. Macromolecules 1981, 14, 915-920 reports the use of terephthalic acid that had been reduced to a particle size of 95% under 10 .mu.m (micrometers) by an air impact method. This fine particle size is required to help overcome the low solubility of terephthalic acid in polyphosphoric acid which limits its rate of reaction. The present invention provides an improved technique for providing the terephthalic acid in a more readily reactive form.